The seasons of the hurricane season

AccuWeather

In today's Forecast Feed, AccuWeather's Bernie Rayno takes a look at the latest on the tropics.

Transcript

00:00Well, we think of the hurricane season as one season that begins in June and ends in November,

00:07but actually there are what I've identified as four seasons within the hurricane season,

00:13and we're in the final season of the hurricane season. Make sense? Take a look at this. This is

00:19hurricane season. Again, sometimes you can get storms before and then after the official end

00:26then start of the season, but by and large, I mean, you look in here, this is the heart of the season,

00:32right? From about August 15th through the month of September. That is the heart of the season.

00:38However, it's a little more complicated than that because I've identified four seasons within the

00:44season. Again, you could see all the storms here that we've had, but what I've identified is this.

00:50The four seasons, early season development is characterized by what? Homegrown development,

00:57right? That's what we see. We also see homegrown development when? Late in the season where we

01:05are right now. And in fact, when you look at the breeding ground for November, you'll notice it's

01:13close to the United States. You see that? It changes quite a bit from October and certainly September

01:19than where it is right now. And the reason we see what we call homegrown development is because the

01:26only way to get development a lot of times this time of the year is with a dip in the jet stream

01:32or an interaction with the jet stream and the tropics. And there's a reason for that. You still

01:37have lots of warm water in the Caribbean or in the Atlantic, but boy, oh boy, wind shear,

01:44wind shear, wind shear, wind shear becomes very detrimental in the Atlantic. In fact, here we go.

01:49We're not even in November or the last day in October. I mean, any tropical wave, look at the

01:56pathway it has to get. Just looking at today, they even make it into the Caribbean or move toward the

02:02United States. Just pockets of strong wind shear denoted by the purple. So for the most part,

02:09your African wave train season is over. That means you've got to look for homegrown development.

02:15development. That's the only way to get development this time of the year. Now, water temperatures are

02:20still warm, but these tropical waves will get shredded by the westerly wind shear across the

02:28Atlantic. Now, for those that you don't know, the term homegrown development is a unique AccuWeather

02:34term. I never heard it any place until I started here 35 years ago. And this is what it entails.

02:40It entails a dip in the jet stream coming across the United States. Here's what it looks like here.

02:48So you get the jet stream southward. And what happens is the waters are still warm. Now you see

02:54this in early season and you also see it in late season. So the way in which we get storms now,

03:00very typical or very similar to how we get storms when? During the early part of the season in June and

03:07July. So you bring this jet stream south and think about it this way. In order to get

03:11the tropical development, you need showers and thunderstorms to form, right? You get that with

03:16tropical waves, but that's off the table. But the jet stream, cooler air coming in over the warm

03:22waters, you bring frontal boundaries and you bring jets, upper lows into this area, right? And that's

03:29how you can start the process of showers and thunderstorms. And then you look at the other

03:33characteristics. What's the wind shear like? What's the water temperature, right? What's the dry,

03:37what does the dry air look like? And then you can get development. So you see that a lot in the early

03:42season and late season. The other way, and it's part of the homegrown development process, what I

03:48should have put in here is you get gyres to form. Okay. That sounds complicated, right? It's really not.

03:53So you get this dip in the jet stream, right? And a gyre forms because you're changing the wind

03:59direction across Mexico and Central America, usually coming in out of the Southeast. But when

04:05you get a dip in the jet stream here, and you need that to occur for about, I'd say, 42 to three days,

04:12you change the wind flow, right? Across North, across Mexico and Central America out of the West,

04:18Northwest. But you keep the easterly flow coming in across the Caribbean. So what you ended up doing is

04:24you start getting this, an anticyclotic wind flow here, a gyre or an area of low pressure.

04:30What happens with low pressure? Upward motion. And then eventually you get an area of showers and

04:37thunderstorms that can then develop into a tropical system. Upper lows, fronts, and gyres all due to the

04:47weather pattern. Okay. So moving forward, do we have any of that going on? Well, remember the

04:53equations haven't changed for tropical development. You need the warm water. We got that. Check. But

04:59these are the two limiting factors during the month of November. And if I would say the one that is the

05:07most detrimental, wind shear, strong winds aloft. Now, let me show you something here, what the wind

05:14shear looks like across even in the Caribbean and the Gulf here. This is our wind shear product here.

05:21So you're looking at the winds in the middle and upper part of the atmosphere and it's color

05:26coded. Where you see the red, there's too much wind shear. Take a look. Here is, here is Florida

05:30here. Gulf of Mexico. This is Monday. Closed down. Too much wind shear. But you'll notice pockets in

05:36here. Southwest Caribbean. Let me go forward as we head into Tuesday and Wednesday. Look at this. Blue.

05:42You look at right in here. The fifth and the sixth. You see this area in here? Almost no wind shear.

05:50Almost none. So that's an area to watch. Now, just because you have low wind shear doesn't mean

05:56you're going to get tropical development. Remember, the way we get it is we need the pattern to produce

06:01it. Well, let's take a look at the pattern. And that's the idea behind the feed. I want you to look

06:06at the modeling and the tools that I use every day. And this is all powered by the AccuWeather Pro

06:11site. So early next week, you'll see this. Look at this dip in the jet stream coming on in. Now,

06:17this is Sunday, Monday, but it leaves Tuesday. So I don't think that dip in the jet stream

06:23is long enough to produce a gyre. But you know what I do see? I see this, an area of high pressure here.

06:29And oftentimes, you look under the belly for development, under the belly of the high. That

06:34tells me that the area to keep an eye on, if we get development, is going to be in this zone,

06:40right in here. And that's the message on the feed. If we get any tropical development next week,

06:45it'll be in the Southwest Caribbean. And that's the feed.

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